Yun-Chih Lee

Enhanced light trapping based on guided mode resonance effect for thin-film silicon solar cells with two filling-factor gratings

An approach of enhanced light-trapping in a thin-film silicon solar cell by adding a two-filling-factor asymmetric binary grating on it is proposed for the wavelength of near-infrared. Such a grating-on-thin-film structure forms a guided-mode resonance notch filter to couple energy diffracted from an incident wave to a leakage mode of the guided layer in the solar cell. The resonance wave coupled between two-filling-factor gratings would laterally extend the optical power and induce multiple bounces within the active layer. The resonance effect traps light in the cell enhancing its absorption probability. A dynamic light-trapping behaviour in solar cells is observed. A photon dwelling time is proposed for the first time to quantify the light-trapping effect. Moreover, the light absorption probability is also quantified. As compared the grating-on-thin-film structure with the one of planar silicon thin film, simulation results reveal that it is 3-fold enhancement in the light absorption within a spectral range of 920-1040 nm. Moreover, such an enhancement can be maintained even the incident angle of near-IR broadband light wave varies up to +/-40 degrees.

Optical interconnect transmitter based on guided-wave silicon optical bench

An optical interconnect transmitter based on guided-wave silicon optical bench is demonstrated. The guided-wave silicon optical bench (GW-SiOB) is developed on a silicon-on-insulator (SOI) substrate. The three-dimensional guided-wave optical paths on the silicon optical bench are realized using trapezoidal waveguides monolithically integrated with 45° micro-reflectors. Such three-dimensional guided-w ave optical paths of SiOB would simplify and shrink the intra-chip optical interconnects located on a SOI substrate. The clearly open eye patterns operated at a data rate of 5 Gbps verifies the proposed GW-SiOB is suitable for intra-chip optical interconnects.

GaN-Based Light-Emitting Diodes With Pillar Structures Around the Mesa Region

This study presents the numerical and experimental demonstrations for the enhancement of light extraction efficiency in nitride-based light-emitting diodes (LEDs) with textured sidewall and micro-sized pillar waveguides (TSMPW) and nano-textured sidewall and nano-pillars (NTSNP) around the mesa. Using hydrothermal ZnO nanorods as the etching hard mask, the authors successfully formed vertical GaN nano-pillars on the mesa-etched regions. It was found that electrical characteristics observed from the proposed LEDs were near the same as the control samples without the pillars. Output power enhancement of LED with TSMPW was about 11% compared with conventional LEDs, and the output power enhancement of LED was greater than 45% upon replacement of TSMPW with the NTSNP structure. The light extraction efficiency enhancement factors of the LEDs with TSMPW and NTSNP structures simulated by finite-difference time-domain analysis were 16.6% and 23%, respectively.

Azimuthally isotropic irradiance of GaN-based light-emitting diodes with GaN microlens arrays

In this paper, the irradiance-modifying concept is proposed by introducing a microlens array on the p-GaN layer of GaN-based light-emitting diode (LED). Every microlens can locally modulate photons emitting from a micro-scaled active region of multiple quantum wells (MQWs) just beneath the microlens. The azimuthally isotropic irradiance from the GaN-based LED with microlens arrays is demonstrated numerically and experimentally. To realize such a novel LED, one-dimensional GaN microlens array with a period of 1.6 microm and a filling factor of 0.64 are fabricated by using dry etching. According to experimental results, the azimuthally isotropic light emission of proposed LED is observed. By using the angular-resolved photoluminescence, its intensity variation corresponding to the azimuth angles is as low as 10% within the angle region of +/-50 degrees.

Flattened Broadband Notch Filters Using Guided-Mode Resonance Associated With Asymmetric Binary Gratings

In this letter, the broadband notch filter exhibiting a flattened bandstop spectral response is experimentally demonstrated for the first time by using the guided-mode resonance associated with the periodic subwavelength structure possessing asymmetric binary grating profiles. The proposed filter consisting of grating and waveguide structures is realized by utilizing a single-layer poly-silicon film deposited on a quartz substrate. The asymmetric grating profile is carried out by dividing the one-filling-factor binary structure into the one with two distinct filling factors within one identical period. A flat bandstop spectral response at the central wavelength of 1.22 mum is obtained under normal incidence for transverse-electric polarization. Its transmittance of bandstop spectrum can be suppressed as low as -20dB within a broad spectral range of 85 nm and its corresponding Deltalambda/lambda is larger than 6%. The spectra under oblique incidence are demonstrated to verify the proposed filter has a high immunity for the angular deviation up to 20deg

SOI-based trapezoidal waveguide with 45-degree microreflector for non-coplanar light bending

SOI-based trapezoidal waveguide with 45° reflector for non-coplanar light bending is proposed and demonstrated. The proposed structures include 45° micro-reflector and silicon trapezoidal waveguide. Due to the SOI-based trapezoidal waveguide with 45° reflector, light wave can be coupled into silicon waveguide easily and have higher coupling efficiency. All of structures are fabricated using a single-step wet etching process. The RMS roughness of waveguide sidewall and 45° micro-reflector is about 30 nm. The coupling efficiency of proposed structure is -4.51 dB, and misalignment tolerance are 42 μm at horizontal direction and 41 μm at vertical direction. The multi-channel trapezoidal waveguide is also demonstrated. This device can transfer the light wave at the same time, and its cross talk is about -50 dB.

Linear Cascade GaN-Based Green Light-Emitting Diodes With Invariant High-Speed/Power Performance Under High-Temperature Operation

We demonstrate the performance of linear cascade green light-emitting diode (LED) arrays suited for use in plastic optical fiber (POF) communications in automobiles or harsh environments. With this three-LED array, driven by the constant voltage bias of an in-car battery output (12 V) we obtain high-speed ( ~ 100-Mb/s eye-opening), high-coupling power (0.9 mW), and a very small variation of coupled power versus temperature [- 0.12%degC-1 at room temperature (RT)] for the whole measured temperature range (i.e., RT to 120degC). Even under high bias current (100 mA) operation, our device can sustain a clear 150-Mb/s eye-opening from RT to 120degC. The static and dynamic measurement results indicate that the speed and power performance of this device are less sensitive to variations in ambient temperature than are those of the red resonant-cavity LEDs utilized for POF communication.

Chip-Level Optical Interconnects Using Polymer Waveguide Integrated With Laser/PD on Silicon

In this letter, we demonstrate a chip-level high-speed optical interconnect, where the optical transmitter/receiver, the polymer waveguides, and the silicon-trench 45° microreflectors are integrated on a single silicon platform. The silicon platform with a silicon trench can provide independent photonic and electrical layers, respectively, for high-speed and low-speed (except high-frequency transmission lines) data transmissions. In order to demonstrate the technical capability of chip-level optical interconnects, the vertical-cavity surface-emitting laser (VCSEL)/photodetector (PD) and the driver/amplifier IC as well as the polymer waveguides combined with the 45° microreflectors are integrated on the electrical and photonic layers of the silicon platform, respectively. The total optical transmission (VCSEL-to-waveguide-to-PD via two 45° microreflectors) is -4.7 dB. The high-speed transmission experiment shows the clear eye opening up to 20-Gbit/s data rate. The bit error rate better than 10-12 for the proposed architecture is also successfully demonstrated. It reveals such chip-level optical interconnects based on the proposed silicon platform with the polymer waveguides is suitable for high-speed data transmission.

III–Nitride-Based Microarray Light-Emitting Diodes with Enhanced Light Extraction Efficiency

In this paper, the enhancement of light extraction efficiency in III–nitride-based light-emitting diodes (LEDs) with an array of microstructures is demonstrated numerically and experimentally at the near-ultraviolet (n-UV) spectral region. Two different microstructures are adopted to study the mechanism, including the shallow etching of microstructures on the indium–tin-oxide (ITO) p-contact and the deep etching of microholes through the InGaN/GaN multiple-quantum-well (MQW) active structure and down to the n-type GaN layer. Multiple point sources with TE and TM polarizations in the MQW region are utilized in the finite-difference time domain (FDTD) analysis to model the behaviour of LED. The simulated beam patterns of both LEDs reveal that the microstructures act as diffraction gratings to extract guided light in the n-type GaN layer. The measured results of beam patterns also verify the prediction in the numerical analysis.

SOI-based trapezoidal waveguide with 45° microreflector for noncoplanar optical interconnect

A silicon on insulator (SOI)-based trapezoidal waveguide with a 45° reflector for noncoplanar optical interconnect is demonstrated. The proposed waveguide is fabricated on an orientation-defined (100) SOI substrate by using a single-step anisotropic wet-etching process. The optical performances of proposed waveguides are numerically and experimentally studied. Transmittance of -4.51 dB, alignment tolerance of ±20 μm, cross talk of -53 dB, and propagation loss of -0.404 dB/cm are achieved The proposed waveguide would be a basic element and suitable for the future intrachip optical interconnects.